The present invention relates to the melt extrusion of a thermoplastic polymer.
The melt extrusion of a thermoplastic polymer to form fibers and nonwoven webs generally involves forcing a molten polymer through a plurality of orifices to form a plurality of molten threadlines, contacting the molten threadlines with a fluid, usually air, directed so as to form filaments or fibers and attenuate them. The attenuated filaments or fibers then are randomly deposited on a surface to form a nonwoven web.
The more common and well known processes utilized for the preparation of nonwoven webs are meltblowing, coforming, and spunbonding.
Meltblowing references include, by way of example, U.S. Pat. Nos. 3,016,599 to Perry, Jr., 3,704,198 to Prentice, 3,755,527 to Keller et al., 3,849,241 to Butin et al., 3,978,185 to Butin et al., and 4,663,220 to Wisneski et al. See, also, V. A. Wente, "Superfine Thermoplastic Fibers", Industrial and Engineering Chemistry, Vol. 48, No. 8, pp. 1342-1346 (1956); V. A. Wente et al., "Manufacture of Superfine Organic Fibers", Navy Research Laboratory, Washington, D.C., NRL Report 4364 (111437), dated May 25, 1954, United States Department of Commerce, Office of Technical Services; and Robert R. Butin and Dwight T. Lohkamp, "Melt Blowing--A One-Step Web Process for New Nonwoven Products", Journal of the Technical Association of the Pulp and Paper Industry, Vol. 56, No.4, pp. 74-77 (1973).
Coforming references (i.e., references disclosing a meltblowing process in which fibers or particles are commingled with the meltblown fibers as they are formed) include U.S. Pat. Nos. 4,100,324 to Anderson et al. and 4,118,531 to Hauser.
Finally, spunbonding references include, among others, U.S. Pat. Nos. 3,341,394 to Kinney, 3,655,862 to Dorschner et al., 3,692,618 to Dorschner et al., 3,705,068 to Dobo et al., 3,802,817 to Matsuki et al., 3,853,651 to Porte, 4,064,605 to Akiyama et al., 4,091,140 to Harmon, 4,100,319 to Schwartz, 4,340,563 to Appel and Morman, 4,405,297 to Appel and Morman, 4,434,204 to Hartman et al., 4,627,811 to Greiser and Wagner, and 4,644,045 to Fowells.
Some of the difficulties or problems routinely encountered with melt extrusion processes are, by way of illustration only, thermal degradation of the polymer, plugging of extrusion dies, and limitations on fiber diameters, throughput, and production rates or line speeds. Fiber diameters generally are a function of the diameter of the orifices through which the polymer is extruded, although the temperature and velocity of the attenuating fluid can have a significant effect. For some applications, fiber diameters of less than about 10 micrometers are desired. Throughput primarily is a function of the melt flow rate of the polymer, while production rates depend in large measure upon throughput. In other words, throughput and production rates generally are dependent upon the viscosity of the molten polymer being extruded. The difficulties and problems just described result largely from efforts to manipulate melt viscosity to achieve desired throughput and/or production rates. Accordingly, there are opportunities for improvements in melt extrusion processes based on improved melt viscosity control.